Noob Question: Inverse Kinematics Not Working
In order for the IK constraint to know that the knee is supposed to bend (even with a pole target), it needs to start slightly bent. In edit mode, move the kneecap slightly forward.

Once you've done this, the knee should bend correctly.
Note that if the IK target FootIK is further away from the hip than the ankle is, the leg will be fully extended and the knee will still not bend. Bring the target bone closer to the leg (touching the ankle) to make it bend.